This invention relates to furnaces and, more particularly, to furnaces for use in oil refineries and petrochemical plants.
There are many variations in the layout, design, and detailed construction of fired heaters and furnaces. In fact, most fired heaters and furnaces are custom engineered for a particular application.
The simplest type of fired heaters (furnaces) comprise the "all radiant" design, in which the entire radiant tubes or coils are arranged vertically along the walls of the radiant section of the combustion chamber. This design is characterized by low thermal efficiency and typically represents the lowest capital investment for a specified duty. The terminology "all radiant" is somewhat of a misnomer since convection currents do exist because of the flow of flue gases through the combustion chamber and such convection currents account for a portion of the total heat absorbed in the radiant section.
Other types of fired heaters (furnaces) have a separate convection section. The residual heat of the flue gases leaving the radiant section is recovered in the convection section primarily by convection. The convection section can increase the thermal efficiency of the fired heater. The first few rows of tubes in the convection section are sometimes referred to as shield tubes or shock tubes.
The principal classification of fired heaters (furnaces), however, relates to the orientation of the heating coil in the radiant section, i.e., whether the tubes are vertical or horizontal.
In the vertical cylindrical radiant fired heater (furnace), the tube coil is placed vertically along the walls of the combustion chamber. Firing is also vertical from the floor of the vertical heater. Vertical heaters are low in cost and require a minimum of plot area but are low in efficiency. Typical duties are from 0.5 to 50 million Btu/hr. The advantages of vertical cylindrical fired heaters (furnaces) are their: (1) high tube area to structural steel weight ratio, resulting in a low cost per Btu absorbed duty, and (2) minimum of plot plan area. The disadvantages of the furnaces are that they require vertical radiant tubes to carry the process stream from areas of high heat flux to areas of low heat flux several times in each pass resulting in inefficient mediocre heat transfer. Also, the coils in vertical cylindrical fired heaters are not self-draining, causing difficulty in hydrocarbon freeing the tubes which often causes the tubes to become coked or otherwise blocked, resulting in extended downtime for furnace maintenance. Furthermore, if one pass of the vertical tubes is filled with liquid, a no flow condition can occur in that pass until sufficient pressure drop exists across the entire furnace to overcome the vertical liquid head in a full tube; this condition has numerous tube ruptures and furnace fires in vertical cylindrical fired heaters.
In helical coiled fired heaters, sometimes referred to as vertical cylindrical helical coiled fired heaters (furnaces), the coils are arranged helically along the walls of the combustion chamber and firing is vertical from the floor. These heaters are low in cost and have drainable tube coils but are low in efficiency. One limitation on these units is that generally only one flow path is followed by the process fluid. Heating duties range from 0.5 to 20 million Btu/hr. The advantages of helical coil cylindrical fired heaters are that they require a minimum of plot plan area and have a low cost per Btu absorbed duty as well as have a countercurrent flow of the in-tube process stream with the flue combustion gases. The disadvantages of the helical coil cylindrical fired heaters are that they are generally limited to one or two radiant passes thereby requiring increased pressure drops. Also, their maximum overall outside diameter is usually limited to about 15 feet due to shipping limitations on trucks and railways. Larger diameter helical coil fired heaters must be shipped by barge which is more expensive. Furthermore, each loop of the radiant coil in helical coiled fired heaters must be welded together at the erection (construction) site which can be very expensive. Fabrication of helical coil is also expensive. Supporting of an helical coil is probably due to irregular coil thermal expansion.
Vertical cylindrical fired heaters (furnaces) with cross flow convection are also fired vertically from the floor but have both radiant and convection sections. The radiant section tube coil is disposed in a vertical arrangement along the walls of the combustion chamber. The convection section tube coils are arranged as a horizontal bank of tubes positioned above the combustion chamber. Typical duty ranges from 10 to 200 million Btu/hr. The vertical cylindrical fired heater with cross flow convection is more thermally efficient than the oil radiant vertical cylindrical fired heater but has many similar disadvantages.
Another type of vertical cylindrical fired heater is that with integral convection. Vertical cylindrical fired heaters (furnaces) with integral convection are vertically fired from the floor with their tube coils installed in a vertical arrangement along the walls. The distinguishing feature of this type of fired heater is the use of added surface area on the upper reaches of each tube to promote convection heating. The surface area extends into the annular space formed between the convection coil and a central baffle sleeve. Medium efficiency can be attained with a minimum of plot area with vertical cylindrical fired heaters with integral convection. Typical duty for this design is from 10 to 100 million Btu/hr.
The arbor or wicked fired heater (furnace) is a specialty design in which the radiant heating surface is provided by U-tubes connecting the inlet and the outlet terminal manifolds. This fired heater is especially useful for heating large flows of gas from the conditions of low pressure drop, such as is employed in a catalyticreformer charge heater. Firing modes are usually vertical from the floor or horizontal between the riser portion of the U-tubes. This design can be expanded to accommodate several arbor coils within one structure. Each coil can be separated by dividing walls so that individual firing control can be attained. In addition, a cross flow convection section can be installed to provide supplementary heating capacity for chores such as steam generation. Typical duties for each arbor coil are about 50 to 100 million Btu/hr.
Another type of vertical fired heater is the vertical tube, double fired heater. In vertical tubes, double fired heaters (furnaces), vertical radiant tubes are arranged in a single row,in each combustion cell. There are often two cells and the tubes are fired from both sides of the row. A more uniform distribution of heat transfer rates and heat flux are accomplished with vertical tube, double fired heaters than in the heaters previously described. Vertical tube, double fired heaters can use multilevel side wall firing to provide maximum control of the heat flux profile along the length of the tubes. The limited number of tubes could be used, making very low firebox utilization. The typical duty range for each cell runs from about 20 to 125 million Btu/hr. Vertical tube, double fired furnaces, however, are very expensive.
In horizontal tube cabin double fired heaters (furnaces), radiant section tube coils are arranged horizontally in the middle of the heater away from the furnace walls. The convection section to the coil's position has a horizontal bank of tubes above the combustion chamber. Normally these tubes are fired vertically from the floor but they can also be fired horizontally by side wall mounted burners located below the tube coil. These fired heaters range from 10 to 100 million Btu/hr. Horizontal tube cabin double fired heaters are more economical and efficient than the other prior art fired heaters discussed above. The coils are self-draining. These fired heaters are somewhat easier to ship and erect because of segmental fabrication. The disadvantages of the horizontal tube cabin double fired heater are that there are bare end walls without any radiant tubes, which substantially decreases the thermal efficiency of the fired heater. Furthermore, horizontal tube cabin double fired heaters require a large plot plan area and have a very low tube to structural steel weight ratio.
The two cell horizontal tube box fired heater (furnace) has a radiant section tube coil positioned in a horizontal arrangement along the side walls and the roof of the two combustion chambers. These fired heaters are vertically fired from the floor and have a typical duty ranging from 100 to 250 million Btu/hr. The advantages and disadvantages of this heater are similar to that of the horizontal tube cabin fired heater.
Another type of horizontal tube fired heater is the horizontal tube cabin fired heater (furnace) with a dividing bridgewall in which a radiant section tube coil is arranged horizontally along the side wall of the combustion chamber and along the hip. The convection section tube coil takes the form of a horizontal bank of tubes positioned above the combustion chamber. A dividing bridgewall between the cells allows for individual firing control over each cell in the combustion chamber. These heaters have a typical duty ranging from 20 to 100 million Btu/hr. The advantages and disadvantages of this fired heater are similar to horizontal tube cabin fired heaters discussed above.
End fired horizontal tube box fired heaters (furnaces) have a radiant section tube coil positioned in a horizontal arrangement along the side walls and the roof of the combustion chamber. The convection section tube coil is also arranged as a horizontal bank of tubes positioned above the combustion chamber. These furnaces are horizontally fired by burners mounted in the end walls. Typical duty ranges for this design are from 5 to 50 million Btu/hr. The advantages and disadvantages of this fired heater are similar to the horizontal tube cabin fired heater.
In the end fired horizontal tube box fired heater (furnace), a side mounted convection section has a radiant section tube coil disposed in a horizontal arrangement along the side walls and the roof of the combustion chamber. The convection section coil, however, is arranged as a horizontal bank of tubes positioned alongside the chamber. The unit is horizontally fired from burners mounted on the end wall. These furnaces are found in many older installations and are very expensive to construct and maintain. Typical duties range from 50 to 200 million Btu/hr. The advantages and disadvantages of this fired heater are similar in many respects to the horizontal tube cabin fired heater.
It is therefore desirable to provide an improved fired heater (furnace) which overcomes many, if not most, of the preceding problems.